Most cosmetic lotions and creams are emulsions of water-based and oil-based phases. An emulsion is a two-phase system consisting of two incompletely miscible liquids, the internal, or discontinuous, phase dispersed as finite globules in the other. Special designations have been devised for oil and water emulsions to indicate which is dispersed and which the continuous phase. Oil-in-water (o/w) emulsions have oil as the dispersed phase in water as the continuous phase. In water-in-oil (w/o) emulsions, water is dispersed in oil, which is the external continuous phase.
Emulsifiers or emollients are generally used to lower surface and interfacial tensions so as to increase the tendency of the emulsion to spread on a particular surface. The compounds also increase the stability of the formulation at particular pH ranges. Other uses for emollients include increasing the deposition of the emulsion onto a particular surface (i.e., such as skin or hair), or increasing the consistency or thickness of the overall product.
In the prior art, preferred emollients for such cosmetics include those compounds which impart the desired properties to the speciific cosmetic composition. An article by Morris et al. entitled "Use of Rabbit Ear Model in Evaluating the Comedogenic Potential of Cosmetic Ingredients," J. Soc. Cosmet. Chem., 34, pp. 215-225, (August, 1983), discloses that several cosmetics and, in particular, certain cosmetic emollients, caused follicular hyperkeratosis when applied to the external ear canal of rabbits. Their results showed that many common emollients were comedogenic, i.e., capable of causing the hyperkeratosis described above, and thus, would not be desirable for many cosmetic compositions due to the possibility of facilitating the formation of acne.
Compounds are classified as comedogenic or non-comedogenic based on their score in a comedogenicity test outlined by Klingman et al., Cosmetic Arch. Derm., Vol. 106, pp. 843-850, the content which is expressly incorporated herein by reference. In this test, non-comedogenic compounds score less than 1.5, out of a possible score of 5, while comedogenic compounds score above 1.5. The lowest values, of course, are preferred for non-comedogenic cosmetic formulations.
While the Morris article discloses a number of vegetable oils which have desirable non-comedogenic properties, these compounds are generally not desirable for use as cosmetic emollients. Vegetable oils are tri-ester (triglycerides) and do not provide the skin feel of mono or diesters. For example, isopropyl myristate is a mono-ester of myristic acid and isopropyl alcohol. This compound is comedogenic and has a light, desirable skin feel. Isopropyl myristate has been formulated into a wide number of cosmetic because of this desirable skin feel property, however, its comedogenic properties have been only recently discovered. As a result, formulators find this ingredient undesirable for most cosmetic formulations.
Applicants have now discovered a wide range of ricinoleic acid esters which are suitable for use as cosmetic emollients while providing non-comedogenicity to the overall cosmetic formulation.